Method and apparatus for controlling an amplifier

ABSTRACT

A method and apparatus are described for controlling the operation of an amplifier, in which, a bias level applied to the amplifier is adjusted so as to generate an output signal having the desired level of peak to mean ratio.

[0001] The present invention relates to methods and apparatus forcontrolling an amplifier according to an envelope ratio of a signal, andapparatus and methods for determining the peak-to-mean envelope ratio ofa signal. In particular, the present invention may be applied to radiofrequency (RF) signals such as used in the UMTS system.

[0002] RF transmissions from mobile telephones of the present generationtypically have a constant envelope shape, and the RF amplifiers operatewith constant power. However, in the next generation of mobiletelephones, it is intended to operate RF transmissions with a variableenvelope. It would be possible to continuously operate the RF amplifierat a high power, sufficient to generate the largest signal output. Forexample, it would be possible to operate an amplifier at a biasproviding a 1 W output capability despite a current output requirementof only 1 mW output power, in order to be capable of providing a 1 Woutput power when required. However, that would be wasteful of power, asa high standby current (bias) would need to be applied to the RFamplifier at all times. Mobile telephones are typically operated usingrechargeable batteries. It is required for the mobile telephone tooperate for as long as possible, both in standby and operating modes,from a single battery charge. Therefore, the power consumption of themobile telephone should be reduced as much as possible.

[0003] Some attempts have been made to adapt the bias (standby powerconsumption) according to the instantaneous value of the output signalenvelope. While this would reduce the power consumption of theamplifier, and extend battery life, it has not been found possible totrack rapid changes in envelope fast enough to allow efficient RFtransmission.

[0004] It is therefore required to provide a method and apparatus forefficiently determining and controlling the power consumption of anamplifier. In an RF system, the transmitter is likely to operate atrelatively low output power, due to the statistical distribution ofpower output in real networks. It is accordingly preferred that themethod and apparatus be particularly low in power consumption atrelatively low output powers.

[0005] According to an aspect of the present invention, a method forcontrolling the operation of an amplifier comprises the steps of:measuring a characteristic of an output signal of the amplifier;measuring a peak value of the characteristic; measuring a time-averagedmean value of the characteristic; comparing the peak value to the meanvalue; providing a demand signal representing a required value of thepeak to mean comparison; and adjusting a bias level applied to theamplifier so as to generate an output signal having the desired level ofthe peak to mean comparison.

[0006] According to an embodiment of the invention, a peak-to-meanenvelope ratio of the signal is determined. The mean value is calculatedover a relatively lengthy period of time, while peak values are detectedand held, again for a relatively lengthy period of time. Therefore, thepeak-to-mean value of the signal changes relatively slowly as comparedto the rate of change of the envelope itself.

[0007] The present invention therefore provides a peak to mean ratiodetector; apparatus for controlling an amplifier, comprising such peakdetector; a method for controlling an amplifier; and a method forobtaining a peak envelope to mean envelope ratio.

[0008] In particular, according to an aspect of the present invention, amethod for controlling an amplifier comprises the steps of: detecting anoutput power envelope of a signal from the amplifier; deriving a firstvalue, indicative of the output power envelope; and deriving second andthird values, respectively indicative of a mean value, and a peak value,of the first value. The second value is subtracted from the third value,to obtain a difference value. A demand signal is supplied and comparedto the difference value, to derive a bias signal. The amplifier iscontrolled according to the bias signal, to achieve a desired peakenvelope to mean envelope ratio in the output signal of the amplifier.

[0009] In an embodiment of the present invention, the first value is alogarithmic representation of the output power envelope; the second andthird values are respectively mean and peak values of the logarithmicrepresentation of the output power envelope, and the demand signal isadapted to operate with the difference value to produce the requiredadjustment to the bias signal, to control the amplifier as required.

[0010] In an embodiment of the present invention, the first value is alinear representation of the output power envelope; the second and thirdvalues are respectively linear representations of mean and peak valuesof the first value, the difference value is a linear representation ofthe difference between peak and mean values of the output powerenvelope; and the demand signal is adapted to operate with thedifference value to produce the required adjustment to the bias signal,to control the amplifier as required.

[0011] In an embodiment of the present invention, the first value is alinear representation of the output power envelope; the second and thirdvalues are respectively logarithmic representations of mean and peakvalues of the first value, the difference value is a logarithmicrepresentation of the ratio of the peak value of the output powerenvelope to the mean value of the output power envelope; and the demandsignal is adapted to operate with the difference value to produce therequired adjustment to the bias signal, to control the amplifier asrequired.

[0012] In an embodiment of the present invention, the first value is anarbitrary polynomial representation of the output power envelope; thesecond and third values are respectively mean, and peak, values of thefirst value, and the demand signal is adapted to operate with thedifference value to produce the required adjustment to the bias signal,to control the amplifier as required.

[0013] The third value may be obtained by passing a signal representingthe first value in succession through a low pass filter and a peakdetector.

[0014] The second value may be obtained by applying a signalrepresenting the first value through an integrator.

[0015] The step of detecting an output power envelope of the signal fromthe amplifier may comprise the step of downconversion prior todetection. According to a further aspect of the present invention,apparatus for controlling an amplifier to achieve a desired peakenvelope to mean envelope ratio in the output signal of the amplifier,comprises a detector for deriving a first value, representative of anoutput power envelope of a signal from the amplifier; a peak detectorfor detecting a peak value of the first value; an averaging means fordetecting a mean value of the first value. A first subtracter isprovided for subtracting the mean value from the peak value, to obtain adifference value. A second subtracter is provided for subtracting thedifference value from a demand signal to produce a bias signal. An inputto the amplifier is provided for receiving the bias signal, to controlthe amplifier according to the difference between the difference valueand the demand signal, so as to achieve a required peak-to-mean ratio ofthe output signal.

[0016] The detector may be a logarithmic detector, the first value maybe a logarithmic representation of the output power envelope, and thedemand signal may be adapted to operate with the difference value toproduce the required adjustment to the bias signal, to control theamplifier as required.

[0017] Alternatively, the detector may be a linear detector, the firstvalue may be a linear representation of the output power envelope, andthe demand signal may be adapted to operate with the difference value toproduce the required adjustment to the bias signal, to control theamplifier as required. In this case, the difference value may representthe difference between the peak value of the output power envelope andthe mean value of the output power envelope.

[0018] Alternatively, the detector may be a linear detector and thefirst value may be a linear representation of the output power envelope.Logarithmic converters may be provided for converting the peak value andthe mean value into logarithmic representations. In this case, thedemand signal is adapted to operate with the difference value to producethe required adjustment to the bias signal, to control the amplifier asrequired. The difference value will represent the logarithm of the ratiobetween the peak value of the output power envelope and the mean valueof the output power envelope.

[0019] Alternatively, the detector may be an arbitrary polynomialdetector, and the first value may be a corresponding arbitrarypolynomial representation of the output power envelope. In this case,the demand signal will be adapted to operate with the difference valueto produce the required adjustment to the bias signal, to control theamplifier as required.

[0020] In any such apparatus, the peak value may be obtained by passinga signal representing the first value in succession through a low passfilter and a peak detector. The mean value may be obtained by applying asignal representing the first value through an integrator.

[0021] A downconverter may additionally be provided for downconvertingthe signal prior to detection of the output power envelope.

[0022] In certain embodiments of the invention, the amplifier is an RFamplifier.

[0023] According to a further aspect of the present invention, a methodfor obtaining a peak envelope to mean envelope ratio measurement of afirst signal comprises the steps of: deriving a second signal indicativeof the envelope of the first signal; deriving a third signal, indicativeof the mean of the second signal; deriving a fourth signal, indicativeof a peak value of the second signal; and subtracting the third signalfrom the fourth signal, to derive a fifth signal, indicative of the peakenvelope to mean envelope ratio of the signal.

[0024] In such a method, the second signal may represent a logarithm ofthe envelope of the first signal, and the fifth signal may represent alogarithm of the peak envelope to mean envelope ratio of the firstsignal.

[0025] The signals may be transmitted as electrical or optical signals.The signals may be transmitted as analogue levels, or may be digitallyencoded, or may be pulse-modulated waveforms.

[0026] The above, and further, objects, characteristics and advantagesof the present invention will become more apparent with reference to thefollowing description of certain embodiments thereof, given by way ofexamples only, in conjunction with the accompanying drawings, in which:

[0027]FIG. 1A represents an input power to output power relationship fora typical linear amplifier;

[0028]FIG. 1B represents a typical envelope waveform; and

[0029]FIG. 2 represents apparatus for controlling an amplifier accordingto the present invention, incorporating a peak to mean detectoraccording to the present invention, operable according to the method ofthe present invention.

[0030] In some radio frequency systems, such as the UMTS system, it isnecessary to provide a signal with a desired peak-to-mean enveloperatio. In order to achieve a higher data rate from a given bandwidth ofthe RF spectrum, it is necessary to employ a signal with a non-constantenvelope, giving rise to a high peak-to-mean ratio.

[0031] In order to generate RF signals with a varying envelope, linearpower amplifiers are required, and these may be set up to provide somelevel of clipping to the output signal. Allowing some degree of clippingallows the amplifier to operate at lower power consumption, givingincreased battery life.

[0032] Linear power amplifiers are known, having a bias input, whichsets the bias point, which in turn determines the current consumptionand so also the maximum output power. A low bias leads to a low peakoutput power. Gain is also affected by bias but less strongly, thusaffecting the mean output power.

[0033] Linear amplifiers are only linear up to a compression point,which is affected by the bias. FIG. 1A shows the relationship betweenoutput power Pout and input power Pin for a linear amplifier withvarious bias points. Curve 101 illustrates the output power to inputpower relationship for a linear amplifier operated at a relatively highbias, while the curve 102 illustrates a corresponding relationship forthe same amplifier operated at a lower bias. Mean input power Pim andmean output power Pom values are shown, along with a peak input powerPip, and corresponding peak output values Pop, Pop′ for low and highbias cases, respectively. In the low bias case 102, the peak-to-meanratio (Pop/Pom) is reduced by compression in the amplifier. The highbias point 101 should then be chosen to provide a higher peak-to-meanratio (Pop′/Pom), and to preserve the integrity of the input signal.However, if the magnitude of the input signal (Pim, and particularlyPip) is reduced, the lower bias 102 would suffice. Current would bewasted if the high bias 101 is chosen to cope with input signels ofgreater magnitude than necessary.

[0034] A required peak-to-mean ratio should be chosen such that theresultant signal may be clipped, sufficiently to achieve maximumefficiency, while not being so far clipped that spectral spillage limitsare exceeded. By selecting a peak to mean ratio, the amplifier bias maybe set to provide just enough capability to amplify an input signal toan output signal with a certain level of clipping, that is, to operateat an optimal power consumption whatever the level of input signal.

[0035] According to an aspect of the present invention, a peak-to-meanvalue of the envelope of the output signal is measured, using a meanvalue obtained over a relatively long period of time, while the peakvalues are detected and held over a relatively long period of time.Therefore, the peak-to-mean values determined according to the methodsand apparatus of the present invention vary relatively slowly. Themethods and apparatus of the present invention do not attempt to varythe bias level of the amplifier according to real-time variations of theRF signal.

[0036] It is therefore possible that the amplifier bias is excessive fora time, following an abrupt reduction in envelope size, or the envelopebias may be undervalued for a time, following an abrupt increase inenvelope size. However, the operation has been found to be satisfactory,with significant reductions in power consumption being achieved. Theapparatus and method of the present invention may provide for resettingof variables if an abrupt change is to be made to the level of the inputsignal to the amplifier. The present invention particularly relates toan efficient way of determining the peak-to-mean ratio of a signalenvelope, and an amplifier controller using the peak-to-mean ratiodetector.

[0037] The peak to mean detector of the present invention is a systemconcept that allows the bias of a linear power amplifier to be set toproduce a demanded peak to mean ratio of the signal at the output of theamplifier. This allows the overall system to minimise its powerconsumption. The sought peak-to-mean ratio control is a long-termcontrol servo that varies the bias based on the history of the outputsignal. It does not aim to control at the modulation rate, and istherefore equally applicable to narrowband and wideband modulationschemes.

[0038] The most important part of the apparatus of the present inventionis the peak-to-mean ratio detector. This is a circuit which can estimatethe peak and mean envelopes of the output signal, and generate a ratioedoutput that can act as a measurement of the peak to mean value of theoutput signal which is used to control the amplifier.

[0039] As is widely recognised, the operation of mathematical divisionis difficult in electronic or other control circuits, whereas processesof addition and subtraction are relatively easy. Therefore, according toone aspect of the present invention, there is provided a method forobtaining the peak-to-mean ratio without performing a divisionoperation.

[0040] According to certain embodiments of the present invention, themathematical operation that logarithm of the ratio of two quantities canbe expressed as a subtraction of the logarithms of the two quantities,is used:

log_(x)(A/B)=log_(x)(A)−log_(x)(B).

[0041] Therefore, the logarithm of the peak-to-mean envelope is equal tothe difference between the logarithm of the peak value of the envelopeand the logarithm of the mean value of the envelope.

[0042] Referring now to FIG. 2 of the drawings, a linear power amplifier10 is to be set by a bias signal 16, determined according to an inputsignal 12 representing a demanded peak to mean ratio, to be applied tothe output signal 14. The demanded ratio is determined by considerationof the level of clipping that is acceptable. This is easily simulated ormeasured in the design of the apparatus and should be only a function ofthe number of channels being generated by the transmitter. There isalways one pilot channel and one data/voice channel in UMTS as aminimum. If more data channels are added, the peak/mean increases and adifferent demanded peak-to-mean ratio is needed. The criterion for theoutput peak-to-mean ratio is that the output signal meets the relevantspectral requirements and also the “vector error magnitude” ormodulation accuracy requirements. A sampling device 15, such as acoupler, directional coupler or resistive tap, diverts some of theoutput signal energy from the amplifier into a peak-to-mean detector 20of the present invention. A downconverter 22 may be provided, dependingon the frequency of the output signal 14, to convert the signal 14 intoa signal 24 of more appropriate frequency.

[0043] The downconverter 22, where provided, may comprise an isolationamplifier 22 a, supplying a signal to a mixer 22 b. The mixer 22 b mayalso receive a local oscillator signal 22 c from local oscillator 22 dto provide a mixed signal 22 e, which is then passed through a band passfilter 22 f of the required frequency band.

[0044] The output of the downconverter, or the output of the amplifier,if of suitable frequency, is then applied to a logamp detector 26. Thisprovides a signal 28 representative of the logarithm of the envelope ofthe signal 14. Suitable logamp detectors are known, for example, as usedin spectrum analysers and radar receivers.

[0045] The signal 28 is then applied to a peak detecting means 30 and toan averaging means 32, in parallel. The averaging means includes anintegrator 34, which receives the signal 28 and outputs a value 40representing the mean value of the logarithm of the envelope of thesignal 14. The peak detecting means typically comprises a low passfilter 36 and a peak detector 38, connected in series in that order.

[0046]FIG. 1B illustrates an example in the value of the envelope e withtime t. Curve 103 illustrates a typical variation of envelope with time,and peak values can be seen, while a mean value could be estimated. Inthe envelope waveform 103, so-called “super peaks” 104 may occur, iepeak envelope values much higher than the normal peak envelope values.These are comparatively rare, have a very fast rise time and are narrow.Because they are rare, it is perfectly possible and usually acceptableto clip them, without degrading performance. The low pass filter 36serves to filter off the size of these narrow peaks (clip the superpeaks), thereby preventing the overloading of the peak detector 38.

[0047] As a subsidiary function, the low pass filter is preferably alsoadapted to eliminate high frequency, e.g. RF, components, to pass onlythe envelope to peak detector 38

[0048] The peak detector 38 then detects a maximum value of the envelopeof the input signal 28, and holds that value as its output signal 42.The peak detector holds this value for a relatively lengthy period oftime. The decay rate of the peak detector should be as slow as possible,while still providing detection of the majority of peaks within theenvelope of signal 14. In a typical RF signal for mobile telephonyapplications, peak values may last only 10 ns or so, while the decayrate should be set slow enough to retain the peak value for severalmilliseconds, a similar time to that used for averaging in the meandetector 34.

[0049] The signal 40 (representing the mean value of the logarithm ofthe envelope of the signal 14), and the signal 42 (representing themaximum value of the logarithm of the envelope of the signal 14) arethen applied to a subtracter 44, which subtracts the value of signal 40from the value of signal 42, and provides a difference signal 46,representing the logarithm of the peak to mean value of the envelope ofsignal 14.

[0050] A second subtracter 48 receives the difference signal 46 andsubtracts it from the demand signal 12, which it also receives. Thedifference between the two signals is output as an error signal 50. Theerror signal 50 is preferably, although not necessarily, then applied toa smoothing circuit (integrator 52), before being used as the biascontrol signal 16. The integrator 52 is there to define the controlcharacteristics, thereby assuring that the loop settles with zero errorwith a non-overshooting response.

[0051] In the above described embodiment, a logarithmic detector 26 isused, that is, the envelope value is detected, and its logarithm derivedat the same time, to directly produce an output signal 28 which isrepresentative of the logarithm of the envelope. The output 46 ofsubtracter 44 therefore represents the difference between the peak ofthe logarithm of the envelope, and the mean of the logarithm of theenvelope. The mean of the logarithm of the envelope is slightlydifferent from the logarithm of the mean of the envelope, and so thevalue of the signal 46 will not exactly represent the logarithm of themean of the envelope. However, this difference may be compensated for byvarying the values of the demand signal 12 applied to request a certainpeak-to-mean ratio.

[0052] In alternative embodiments, the envelope detection function maybe separated from the logarithm derivation. For example, a diodedetector may be used to provide a linear indication of the envelope,with a logarithmic converter applied later. The logarithmic convertermay be placed between the diode detector and the peak and meandetectors. Individual logarithmic converters may be provided for eachdetector, or a single logarithmic converter may provide a single outputthat is directed to both peak and mean detectors. In further alternativeembodiments, individual logarithmic converters may be placed after eachof the peak and mean detectors, to convert linear values of peak andmean into logarithmic values. In these embodiments, the signal 46produced by the subtracter 44 will represent the actual logarithm of thepeak-to-mean ratio.

[0053] In yet further embodiments of the present invention, logarithmicconversion may be avoided altogether. For example, diode conversion maybe used directly with a peak detector and a mean detector, and peak andmean envelope values applied to the subtracter. The signal 46 suppliedby the subtracter will no longer represent the peak to mean ratio, butwill rather represent a difference between peak value and mean value.

[0054] The principles of the present invention apply whether thedifference signal 46 produced by the subtracter represents a logarithmof the peak to mean ratio, or a difference of the logarithms of the peakand mean values, or a difference of some other function. The demandsignal 12 applied to second subtracter 48 must be adapted to correspondto the type of difference signal in use.

[0055] For example, the demand signal 12 may represent the logarithm ofthe required peak-to-mean envelope, to allow a direct comparison withthe difference signal 46. Alternatively, a linear demand signal may beprovided to a logarithmic converter (not shown) before being applied 12to the subtracter 48. A further alternative would be to convert thedifference signal 46 into a linear signal before applying it to thesubtracter 48, and using a linear demand signal 12.

[0056] Some logarithmic converters, such as the logamps referred toearlier, may be difficult to stabilise in temperature. When used toderive the envelope, temperature stabilisation is not important asdifferential pairs are used, and are relatively immune to temperaturevariation. However, when used as a standalone logarithmic converter,temperature stabilisation becomes important.

[0057] In a second, alternative, embodiment the peak detection means 30and the averaging means 32 each include a logamp detector, similar tothat indicated at 26, placed upstream of the components illustrated inthe drawing. Logamp converter 26 would not then be required.

[0058] In a third embodiment, the logamp detectors referred to inrespect of the second embodiment are located downstream of therespective components illustrated in the drawing. In this embodiment,the subtracter is supplied with the logarithm of the mean of theenvelope, and the logarithm of the peak of the envelope, whereas in theother embodiments, the subtracter is supplied with the mean of thelogarithm of the envelope, and the peak of the logarithm of theenvelope. In such an embodiment, the value of the demand signal 12 hasto be chosen correctly. It should be related to the logarithm of thedifference rather than the simple difference of the logarithms. This isstill just a simple voltage. There is a slight increase in sensitivityto the demand input in this case, since the error voltage is smaller.

[0059] While the present invention has been described with reference toa limited number of particular embodiments, various modifications andadjustments may be made within the scope of the present invention. Forexample, although the present invention is particularly applicable to RFsignals, it may be applied without substantial modification to signalsof other wavelengths.

[0060] The preceding description refers only to “logarithms”, thepresent invention will perform equally well using logarithms to base 10,base e, base 2, base 16 or any other base. For this reason, the formularecited above is expressed in logarithms to base x. As discussedearlier, the present invention may also be applied to arrangements nothaving a logarithmic function.

[0061] The signals referred to above may be embodied as electrical,optical, mechanical or other suitable signals in a circuit designed torespond to such signals. The signals may be expressed as an analoguevalue (e.g. a steady voltage, a steady light intensity, a steadypressure), or by a digital coding of some sort (e.g. hexadecimalnumbering) or a pulse code system (e.g. where the value expressed isrelated to the instantaneous frequency of digital impulses). It may benecessary to convert between types of signal for processing (e.g.optical or mechanical signals 14 may need to be converted intoelectrical signals before they can be processed).

1. A method for controlling the operation of an amplifier, comprising the steps of: measuring a characteristic of an output signal of the amplifier; measuring a peak value of the characteristic; measuring a time-averaged mean value of the characteristic; comparing the peak value to the mean value; providing a demand signal representing a required value of the peak to mean comparison; and adjusting a bias level applied to the amplifier so as to generate an output signal having the desired level of the peak to mean comparison.
 2. A method for controlling an amplifier (10) according to claim 1, comprising the steps of: detecting (15,26) an output power envelope of a signal from the amplifier; deriving a first value (28), indicative of the output power envelope; deriving second (40) and third (42) values, respectively indicative of a mean value, and a peak value, of the first value; subtracting (44) the second value from the third value, to obtain a difference value (46); supplying a demand signal (12) and comparing (48) it to the difference value, to derive a bias signal (16); and controlling the amplifier according to the bias signal, to achieve a desired peak envelope to mean envelope ratio in the output signal of the amplifier.
 3. A method according to claim 2, wherein the first value is a logarithmic representation of the output power envelope; the second and third values are respectively mean and peak values of the logarithmic representation of the output power envelope, and the demand signal is adapted to operate with the difference value to produce the required adjustment to the bias signal, to control the amplifier as required.
 4. A method according to claim 2, wherein the first value is a linear representation of the output power envelope; the second and third values are respectively linear representations of mean and peak values of the first value, the difference value is a linear representation of the difference between peak and mean values of the output power envelope; and the demand signal is adapted to operate with the difference value to produce the required adjustment to the bias signal, to control the amplifier as required.
 5. A method according to claim 2, wherein the first value is a linear representation of the output power envelope; the second and third values are respectively logarithmic representations of mean and peak values of the first value, the difference value is a logarithmic representation of the ratio of the peak value of the output power envelope to the mean value of the output power envelope; and the demand signal is adapted to operate with the difference value to produce the required adjustment to the bias signal, to control the amplifier as required.
 6. A method according to claim 2, wherein the first value is an arbitrary polynomial representation of the output power envelope; the second and third values are respectively mean, and peak, values of the first value, and the demand signal is adapted to operate with the difference value to produce the required adjustment to the bias signal, to control the amplifier as required.
 7. A method according to any of claims 2-6, wherein the third value (42) is obtained by passing a signal representing the first value (28) in succession through a low pass filter (36) and a peak detector (38).
 8. A method according to any of claims 2-7, wherein the second value (40) is obtained by applying a signal representing the first value (28) through an integrator (34).
 9. A method according to any of claims 2-8, wherein the step of detecting an output power envelope of the signal from the amplifier comprises the step of downconversion (22) prior to detection.
 10. Apparatus for controlling an amplifier (10) to achieve a desired peak envelope to mean envelope ratio in the output signal of such amplifier, comprising: a detector (26) for deriving a first value (28), representative of an output power envelope of a signal (14) from such amplifier; a peak detector (30) for detecting a peak value (42) of the first value; an averaging means (32) for detecting a mean value (40) of the first value; a subtracter (44) for subtracting the mean value from the peak value, to obtain a difference value (46); a subtracter (48) for subtracting the difference value from a demand signal (12) to produce a bias signal (16) for controlling such amplifier according to the difference between the difference value and the demand signal, so as to achieve a required peak-to-mean ratio of the output signal.
 11. Apparatus according to claim 10, wherein the detector is a logarithmic detector, the first value is a logarithmic representation of the output power envelope, and the demand signal is adapted to operate with the difference value to produce the required adjustment to the bias signal, to control the amplifier as required.
 12. Apparatus according to claim 10, wherein the detector is a linear detector, the first value is a linear representation of the output power envelope, and the demand signal is adapted to operate with the difference value to produce the required adjustment to the bias signal, to control the amplifier as required, wherein the difference value represents the difference between the peak value of the output power envelope and the mean value of the output power envelope.
 13. Apparatus according to claim 10 wherein the detector is a linear detector and the first value is a linear representation of the output power envelope, and wherein logarithmic converters are provided for converting the peak value and the mean value into logarithmic representations and wherein the demand signal is adapted to operate with the difference value to produce the required adjustment to the bias signal, to control the amplifier as required, wherein the difference value represents the logarithm of the ratio between the peak value of the output power envelope and the mean value of the output power envelope.
 14. Apparatus according to claim 10, wherein the detector is an arbitrary polynomial detector, the first value is a corresponding arbitrary polynomial representation of the output power envelope, and the demand signal is adapted to operate with the difference value to produce the required adjustment to the bias signal, to control the amplifier as required.
 15. Apparatus according to any of claims 10-14 wherein the peak value is obtained by passing a signal representing the first value in succession through a low pass filter (36) and a peak detector (38).
 16. Apparatus according to claim 14 or claim 15, wherein the mean value is obtained by applying a signal representing the first value through an integrator (34).
 17. Apparatus according to any of claims 10-16, wherein a downconverter (22) is provided for downconverting the signal prior to detection of the output power envelope.
 18. A method or apparatus according to any preceding claim, wherein the amplifier is an RF amplifier.
 19. A method or an apparatus according to any preceding claim, wherein the signals are transmitted as electrical or optical signals.
 20. A method or an apparatus according to any preceding claim, wherein the signals are transmitted as analogue levels, or are digitally encoded, or are pulse modulated waveforms. 